Pseudomembranous colitis is a life-threatening disease that is caused indirectly by antibiotic therapy. Physicians now are forced to use large doses of antibiotics to eliminate increasingly resistant pathogens, and such therapy kills many of the beneficial anaerobic bacteria in our colons. In this altered ecosystem, other pathogens can grow and produce toxins that can kill the patient. Clostridium difficile is the most common cause of this antibiotic-associated colitis. This species produces an enterotoxin (toxin A) and a potent cytotoxin (toxin B) both of which appear to be involved in the disease. We have purified both toxins and shown that they are composed of subunits. We will isolate the subunits and define the toxic and binding subunits (if they are different). We have identified a glycoprotein receptor for the enterotoxin, and we also will isolate this receptor. The carbohydrate specificity of the binding site will be determined, and we will define whether this receptor is involved in internalization of the toxin. We will examine the binding of the toxins in vivo to the hamster's colon and in vitro to brush border membranes. The receptor for toxin A will be used in rapid latex agglutination and passive hemagglutination tests to detect the toxin in fecal specimens. Our monoclonal antibodies to toxin A also will be used in such assays, and we will develop monoclonal antibodies for toxin B. We also are developing ELISA tests for other clostridial toxins that may cause antibiotic-associated diarrheas. Once such toxin is the iota toxin of C. spiroforme, which we have found to consist of two synergistic proteins. Toxins A and B of C. difficile also appear to act synergistically, and we will examine the mechanisms involved. Our research will result in a better understanding of colitis and diarrhea resulting from antibiotics and should lead to improved diagnosis.